LADAR (laser detecting and ranging) range finders, much like their RADAR (radio detecting and ranging) counterparts, determine the distance to a target by measuring the time interval required for an energy, or ranging, pulse to be transmitted to the target and reflected back to the laser. The time interval between transmission of the ranging pulse and reception of the reflected ranging pulse is a measure of the distance, or range, to the target.
Conventional laser range finders typically use an electronic range counter circuit to measure the time interval. The range counter circuit generally includes a high frequency, digital range clock for producing clock pulses, a range counter for counting the clock pulses, and a range latch for storing the total integer number of clock pulses counted by the range counter. The resolution of the range measurement thus depends, to a great extent, on the precision of the range counter circuit and the operating frequency of the range clock.
The range counter counts the integer number of clock pulses produced by the range clock during the time interval between transmission of the ranging pulse and reception of the reflected pulse. The range counter begins counting at the first clock pulse after the ranging pulse is transmitted, and stops counting at the first clock pulse after the reflected ranging pulse is received. The total integer number of clock pulses counted during the time interval is a function of the measured range to the target. The true range, however, is almost always different from the measured range. The error between the measured range and the true range depends on the synchronization of the clock pulses with the transmission and reception of the ranging pulse.
FIG. 1 illustrates two examples of a typical error between the measured range and the true range. The square waveform shown at the top of FIG. 1 represents the digital clock pulses produced by the range clock. The waveform transitions to a positive level each time the range clock pulses. The examples labeled "A" and "B" in the middle and at the bottom of FIG. 1, repectively, each represent a ranging pulse transmitted by the laser, reflected by the target, and received back at the laser.
In example "A", an electronic start pulse simultaneously signals the laser to transmit the ranging pulse and the range counter circuit to begin counting range clock pulses. The ranging pulse is transmitted (in response to the start pulse) just before the clock pulse labeled "C1" near the top of FIG. 1. Thus, the first clock pulse counted by the range counter is counted at the positive transition of the first clock pulse after the start pulse is received and the ranging pulse is transmitted as indicated by "C1".
The ranging pulse is reflected by the target and received back at the laser. An electronic stop pulse is transmitted to the range counter circuit when the reflected ranging pulse is received back at the laser. In example "A", the reflected ranging pulse is received back at the laser just after the third clock pulse (indicated by "C3") is counted. The stop pulse signals the range counter circuit to stop counting clock pulses. Thus, the last clock pulse counted by the range counter is counted at the positive transition of the first clock pulse after the reflected ranging pulse is received and the stop pulse is transmitted as indicated by "C4".
The measured range in example "A" is therefore equal to 4 clock pulses. The true range, however, is only equal to approximately 2 and 1/4 clock pulses. The error between the measured range and the true range is thus about 1 and 3/4 clock pulses.
In example "B" at the bottom of FIG. 1, another ranging pulse is transmitted by the laser, reflected by the target, and received back at the laser. The ranging pulse in example "B" is transmitted (in response to the start pulse) just after the clock pulse labeled "C1" near the top of FIG. 1, and before the clock pulse labeled "C2". Thus, the first clock pulse counted by the range counter is counted at the positive transition of the first Clock pulse after the start pulse is received and the ranging pulse is transmitted as indicated by "C2".
As in example "A", the ranging pulse is reflected by the target and received back at the laser. An electronic stop pulse is again transmitted to the range counter circuit when the reflected ranging pulse is received back at the laser. In example "B", the reflected ranging pulse is received back at the laser just before the fourth clock pulse (indicated by "C4") is counted. The stop pulse signals the range counter circuit to stop counting clock pulses. Thus, the last clock pulse counted by the range counter is counted at the positive transition of the first clock pulse after the reflected ranging pulse is received and the stop pulse is transmitted as indicated by "C4".
The measured range in example "B" is therefore equal to 3 clock pulses. The true range, however, is only equal to approximately 2 and 3/4 clock pulses. The error between the measured range and the true range is thus about 1/4 clock pulses.
As FIG. 1 illustrates, the total error between the measured range and the true range can vary in magnitude between 0 and 2 clock pulses. If the time between transmission of the ranging pulse and the next clock pulse is small, as illustrated in example "A", the measured range is greater than the true range by an amount approaching 1 clock pulse. The error approaches 1 clock pulse as transmission of the ranging pulse and the positive transition of the next clock pulse approach coincidence.
If the time between transmission of the ranging pulse and the next clock pulse is large (up to one period of the clock pulse) as illustrated in example "B", the measured range is greater than the true range by an amount approaching 0 clock pulses. The error approaches 0 clock pulses as transmission of B the ranging pulse and the positive transition of the previous clock pulse approach coincidence.
The measured range is also greater than the true range by an amount approaching 1 clock pulse if the time between reception of the ranging pulse and the next clock pulse is large (up to one period of the clock pulse), as illustrated in example "A". The error approaches 1 clock pulse as reception of the ranging pulse and the positive transition of the previous clock pulse approach coincidence.
Similarly, the measured range is also greater than the true range by an amount approaching 0 clock pulses if the time between reception of the ranging pulse and the next clock pulse is small, as illustrated in example "B". The error approaches 0 clock pulses as reception of the ranging pulse and the positive transition of the next clock pulse approach coincidence. The total error between the measured range and the true range thus can vary in magnitude between 0 and 2 clock pulses.
The asynchronous nature of transmission and reception of the ranging pulse relative to the clock pulses results in "synchronization jitter" in the range measurement. Transmission synchronization error results in a measured range value greater than the true range value which can vary in magnitude between 0 and 1 clock pulse, or Least Significant Bit (LSB), of the range counter. Reception synchronization error results in a measured range value greater than the true range value which also can vary in magnitude between 0 and 1 LSB. Example "A" of FIG. 1 illustrates the case in which both synchronization errors approach 1 LSB, and the total error approaches 2 LSBs. Example "B" of FIG. 1 illustrates the case in which both synchronization errors approach 0 LSBs, and the total error approaches 0 LSBs.
A typical frequency of a digital range clock for a conventional laser range finder is about 60 megahertz (MHz). Because the maximum pulse repetition rate in a typical 60 MHz range counter circuit is on the order of 30-40 MHz, the resolution of a laser range finder having a 60 MHz range clock is limited to approximately 2.5 meters per clock pulse. A total error of between 0 and 2 LSBs of the range counter circuit therefore corresponds to a random error of between 0 and 5 meters in the range measurement.
The obvious solution to enhance the resolution of a range finder is to increase the frequency of the range clock. A 60 MHz frequency for the range clock, however, is standard in the industry and is compatible with most of the electronic circuits which may be used in conjunction with a conventional range finder.
More importantly, there is a practical limit to the frequency of the range clock. Manufacturing technology limits the physical size of the etch of an integrated circuit to about 0.5 microns. In addition, because the specific impedence of the capacitance is inversely proportional to the frequency, as the frequency increases, the impedence of the circuit decreases. Therefore, the circuit is more suceptible to high frequency electromagnetically radiated energy. Accordingly, it is not practical to increase the operating frequency without excessive shielding or high frequency methodolgy becuause of the disadvantages associated with a higher frequency range counter circuit and range clock.
It is therefore an object of the invention to provide a range finder which enhances the resolution of the range measurement without increasing the frequency of the range counter circuit and the range clock.
It is a more particular object of the invention to provide an electronic vernier for a range finder which reduces the error between the measured B range and the true range to a target without increasing the frequency of the range counter circuit and the range clock.
It is another object of the invention to provide a range finder which includes a range clock having an operating frequency of 60 MHz.
It is another object of the invention to provide a range finder which includes a range counter circuit and a range clock which do not operate at a frequency which will cause electromagnetic coupling due to capacitive effects and thus make adjacent electronic circuits more suceptible to radiated high frequency noise.